Induction heating apparatus



y 1944. P. E. DRAVNEEK 2,353,130

mnucmn HEATING APPARATUS Filed April 14, 1945 STEP-D O WN W a TRA NSF'ORMER SECONDARY INVENTOR. F34 u1 EDRA v/vEEK.

ATTORNEYS.

Patented July 11, 1944 UNITED STATES PATENT orricsmnuc'rron naA'rmG APPARATUS rm 1:. Dravneek, New York, N. Y., minorto Induction Heating Com, New York, N. Y., a corporation of New York Application A911! 14, 1943, Serial No. 482,969

'I' Claims. (Cl. 219-13) This invention relates to induction heatin apparatus and more particularly to devices, or so-called work coils, for applying the high frequency field to the objects to be heated.

This application comprises a continuation in part of my conending application Ser. No. 470,233, filed December 26, 1942, now Patent No. 2,321,189,

granted June 8, 1943, entitled Induction heating apparatus.

For the efllcient use of induction heating apparatus in manufacturing operations, it is oftentimes desirable to concurrently heat a group or series of articles with the same work coil. Many of the problems in induction heating are such as to require an accurately predetermined and uniform amount of heating of all of such a series of objects. A convenient way to heat such groups or series of articles is to place them in alignment, supported for example by a suitable fixture, and to then apply a conductor carrying the high frequency current, so as to extend from the source of power along one side of the series of objects, then reversing and extending back along the other side of the series, whereby all of the articles are more or less surrounded, in closely spaced inductive relationship to a single work coil turn. Or if desired, the work coil in such elongated form may include two or more turns.

However, when a series of objects are heated in the manner above described, even though the sides of all the objects are substantially uniformly spaced from the high frequency conductor or turn, there will be considerable variation in the amount of heat generated in the objects at different positions along the series. For example, the end object of the series will be subjected to considerably more heating due to the reversing end portion of the turn which causes a substantially more concentrated field in this region. And if the series is very long, the induction and capacity effects with the high frequencies used, will cause further variations in the heating along the series.

The voltage drop across the terminals of a work coil such as above referred to, may amount for example to several thousand volts. Accordingly, an elongated work coil turn as applied to a series of objects, involves the further diificulty that very closely adjacent points on the turn may be subject to a high potential diflerence, dangerous for the workman and liable to cause destructive arcing upon short-circuiting of such points by contact with the work or otherwise.

The present invention provides means for overcoming the above noted difficulties in various in ways, and also makes possible a simple, inexpensive, dependable and safe work coll construction and arrangement.

Various further and more specific objects, features and advantages of the invention will appear from the detailed description given below taken in connection with the accompanying drawing which forms a part of this specification and illustrates merely by way of example,'preferred forms of the invention. The invention consists in such novel features and combinations of parts as may be shown and described in connection with the apparatus herein disclosed.

In the drawing,

Fig. 1 is a plan view showing one of the preferred embodiments of the invention;

Fig. 2 is a vertical sectional view taken substantially along line 2-2 of Fig. 1; and

Fig. 3 is a view illustrating another embodiment of the invention.

Referring more specifically to Figs. 1 and 2, portions of a high frequency step-down transformer are indicated at It, the secondary terminals of the transformer being shown at II, II. In this case a one turn work coil I3 is constructed of a T-shape andarranged to be supported if desired directly upon and by the transformer secondary terminals. Starting at the terminal ii, the divided base portion of the T is formed with a pair of attaching lugs or terminals as at i4, IS. A heavy conductor comprising for example a rigid bar of copper of rectangular cross section may have an angular end portion as at It brased to the lug ll, such bar'extending out as at H to the cross portion of the T and then extendin with a generally scalloped formation as at l8, out to one outer end of the cross portion. The bar then reverses and extends back with a generally complementary scalloped formation as at llto the middle 20 of the top'of the T. The inwardly protruding portions of the complementary scalloped formations approach each other quite closely as at 2! so that they are spaced apart only by narrow gaps or slots of the width for example of a sawcut. The above described formation thus provides a series of openings as at 22, 23 for receiving the objects to be heated. As shown, such objects may comprise cylindrical members as at 24, although it will be understood that they may be of polygonal or other shapes, in which event the scalloped formations will be correspondingly altered in shape so as to substantially conform in closely spaced inductive relation to the peripheries of the objects. As shown in Fig. l, the righthand portions ofthe T-shaped device may preferably be made symmetrical with the above described lefthand portions, the terminal lug ll being connected to the righthand portions by a part 2| of the conductor bar, which may as shown form an integral continuation of the same conductor which starts at l8, l1 and passes around the series of objects and back to the transformer. The leg or supporting portion of the T thus comprises the two portions I! and which are separated by a narrow slot 26 which opens into a slot 21 at the middle of the cross portion of the T. The slots 2|, 26 and 21 are all made quite narrow in order to bring opposite portions of the conductor turn closely adjacent, to thereby prevent excessive induction therebetween with consequent choking oil of the high frequency current from the positions on the coil more remote from the transformer.

The narrow gaps 2i also make it possible to substantially unif'ormly surround the objects :4 with the desired uniform high frequency field.

In order to provide the necessary cooling of the conductor turn, a metal cooling fluid conduit as at so may be secured as by brazing or soldering along the entire length at the outside surface of the conductor turn. To avoid angular bends and obstructions, this conduit as shown may bridge across the more abrupt portions of the scallopped formations, etc. This at the same time serves to give the main conductor greater rigidity and strength particularly at the angular portions most liable to breakage or bending. Also as shown at II, the cooling fluid conduit bridges and strengthens the angular formation adjacent the cross-over of the T.

In the form of the invention shown in Fig. 1, several expedients are used for overcoming the above-mentioned excess heating effect which would otherwise occur on the end objects as at 24' of the series being heated. Since the reversing end portion of the conductor as at I! in effect provides a terminating loop around the object 24', the field strength in the region of this reversing portion will be substantially increased. I have found it possible to eifectively position this region of increased neld strength further from the end object 24' by cutting a notch or slot 33 in the reversing end portion, at a point preferably in general alignment with the slots I I. This serves to space the region of reversal of the conductor, and the consequent area of concentration of the lines of force, away from the end object and thereby considerably reducing and sometimes elirninating the difiiculty of overheating the end ob ect.

Usually, however, to insure the desired result, it is advisable to resort to a further expedient, i. e., that of forming the end opening as at it somewhat larger than the other openings as at 22. Then assuming that all the objects are alike, the walls within the end opening will in general be spaced further from the periphery of the end object than is the case at the other objects. Since the degree of heating imparted to the object is.

reduced in general, according to a square law, upon increasing the spacing, it will be apparent that by increasing the .size of the opening 23,

one may readily compensate for the increased heating effect on the end object as a whole.

However. this will still not fully solve the problem in cases where it is desired to have substantially uniform heating all around the periphery of the end object. That, is, assuming the end object is placed centrally of the end opening 28,

sometimes be sumciently overcome by the use of a slot or notch as at 33 of requisite depth, it has been usually found preferable for the purpose to also so position the end object 24' in its opening 23 that the spacing a will be greater on the outer side than on the inner side at b, as is shown in Fig. 1. Assuming the objects 24, 24' are all alike, the spacing b for example may be about the same as the normal spacing around the periphery of the other objects 24. In practice it has been found possible to readily determine by trial the necessary amount of increase in size of the opening 23 as compared with openings 22, also the proper relative spacings a and b and depth of notch 33, for obtaining the proper amount of heat in the end object as a whole, as well as for applying such heat uniformly around the periphery of the object.

In Fig. 2 a suitable removable fixture is indi cated at I! for supporting and carrying the objects 24, 24' in proper spaced relationship for insertion and removal into and from the desired positions in the openings, as above described.

This fixture may of course be put into place or removed manually or by suitable automatic mechanism, and permits the workman to load one fixture with a series of objects while another fixture with objects thereon is in heating position, whereupon the two fixtures may be quickly interchanged.

The above described device of T-shape has a number of important advantages, particularly in cases where a series of 6, 8 or more objects is to be heated. In such cases the T-shape permits the high frequency current to be conducted to and from the middle of the series, thereby insuring that the objects on one side of the T will be heated to the same extent as those on the other side of the symmetrical arrangement. Without such a T formation and if the high frequency current terminals for example, were located at one end of such a long series of objects, due to inductance and capacity eflects along the turn, the outer and inner objects of the series might not be heated to the required uniform degree. The 1'' formation also provides a more rigid structure well adapted to be supported centrally by the leg portion of the T. The leg portion also provides short and direct outgoing and return connections to the transformer, while permitting the objects to be placed along a line facing the position of the workman so that he does not have to reach back over or under any inaccessible part of the device when using same.

In l lg. 3 a straight form of one-turn coil is shown at 40 for extending directly out from a pair of transformer terminals II', II. This form of the device is sometimes preferable as in cases where a small and odd number of objects are to be heated. In the example shown in Fig. 3, the coil has three openings, ll, 12, 0, connected by slots as at H, the supporting portions which run back to the transformer terminals being separated by a narrow slot 4| opening into opening ll. At the outer side of opening 43 a notch 33' may be provided for the same purpose as notches II in Fig. 1. In fact the same general principles of construction may be utilized in the device of Fig. 3 as above described in connection with the or with uniform spacing all around, then the in-- lefthand end portion of Fig. 1. In Fig. 3 the 2,ass,1so

, high frequency conductor, instead of comprising a bar of metal bent or cast with the scalloped formation of Fig. 1, may here if desired comprise merely a slab of metal, such as copper, with the openings "-43 formed by drilling or otherwise, and with the slots provided by a sawcut. Here, it will be noted, the inner and active conductor surfaces or portions also effectively extend with a scalloped formation, along one side ,of the series of objects, then reversing to extend back again with a generally complementary scalloped formation along the other side of the series of objects. As in the case of Fig. 1, a cooling fluid conduit 3|).may be brazed or soldered to r extend around the periphery of the device.

In Fig. 3 the secondary turns of a high frequency step-down transformer are shown at 46 connected to the terminals II and I! to thereby complete the work circuit. The work crcuit for the device of Fig. 1 may be completed in a like way. The secondary transformer as well as its terminals, may be in the form of cooling fluid conduits, the terminals if desired being arranged for the flow of a stream of cooling fluid in series through the secondary and around the work coil member through conduit 30' as shown.

As above stated, the drop across the terminals of a device of this kind, may amount to several thousand volts, and the drop with respect to ground at any point on the circuit may be also quite high unless such point is grounded. It is generally the practice to ground the objects being heated, or the fixture for holding same, or if the objects and fixture are insulated from ground,

there will still be considerable capacity effect with respect to ground. Also it has been here-- tofore customary to ground one of the work coil terminals. But in that case there will occur between the portion of the work coil nearest its ungrounded terminal and the adjacent grounded object or objects, a voltage drop almost as high as the several thousand volts across the terminals. Thus if the objects get out of place and touch such portions of the work coil or if shortcircuiting or arcing occurs due to filling the narrow coupling gap with brazing flux for example, the resulting arcing may be quite destructive. Also if one of the work coil terminals is grounded, the voltage drop with respect to ground of the outer portion of the devices nearest the workman will be very substantial and present a hazard. To greatly minimize these difilculties, according to the present invention the outer middle point on the work coil turn or device is grounded as by a ground connection 41 in Fig. 1, Or 48 in Fig. 3, these connections providing the sole grounding for the work coil circuit in each case. With the grounds in these positions, the portions of the work coils nearest the workman may be safely touched without danger and the highest voltage drops are greatly reduced as between each of the objects and the adjacent portions of the work coil. Consequently the danger that arcing will start between the grounded objects and the work coil is considerably reduced and furthermore if arcing does start, its destructiveness is also considerably reduced.

While the invention has been described in detail with respect to particular preferred examples, it will be understood by those skilled in the art after understanding the invention that various changes and modifications may be made without departing from the spirit and scope of the invention, and it is intended therefore in the appended claims to cover all such changes and modifications.

What is claimed as new and desired to be secured by Letters Patent is:

1. 1m inductor for applying a high frequency electromagnetic field to a plurality of objects to be heated, comprising conductor means for extending with a scalloped formation along one side of said plurality of objects and then reversing to extend back again with a generally complementary scalloped formation along the other side of said plurality of objects, the protruding portions of the scalloped formation on one side being separated by narrow gaps from the corresponding portions on the other side, whereby a plurality of openings is provided with their walls adapted for closely spaced coupling relationship to the peripheries of the objects and for encircling the objects except at said gaps, the reversing end portion of the conductor means forming a loop for surrounding the end object, the inside surface of said reversing portion being formed with a narrow notch to thereby cause the region of increased field strength due to said reversing portion to be located in a more spaced relation to the end object.

2. In induction heating apparatus, the combination of a fixture and an inductor for applying a high frequency electromagnetic field to a series of at least several like objects to be heated, said inductor comprising conductor means for extending with a scalloped formation along one side of said series and then reversing to extend back again with a generally complementary scalloped formation along the other side of said series, the

protruding portions of the scalloped formation on one side being separated by narrow gaps from the corresponding portions on the other side, whereby a seriesbf openings is provided with their walls adapted for closely spaced coupling relationship to the peripheries of the objects and for encircling the objects except at said gaps, the reversing end portion of the conductor forming a loop for surrounding the end object of the series, the opening in said loop being formed larger than the others of said openings, to an extent suflicient to increase the coupling spacing from the periphery of the end object by an amount substantially compensating for the greater field strength due to said loop, and said fixture being constructed and arranged for holding the objects in said openings, with generally uniform spacing between said walls and said -peripherles, except for the end object, and for holding the end object in its opening with a somewhat greater spacing from said reversing end portion, to thereby provide for substantially the same amount of heat in the end object as in each of the others and for generally uniform heating around the periphery of the end object.

3. In induction heating apparatus, a device for applying a high frequency electromagnetic field to a series of objects to be heated, comprising rigid substantially T-shaped conductor means formed with a series of openings along the cross portion of the T, for respectively receiving the objects, said openings being jointed by narrow slots, the leg of the T being divided by a narrow longitudinal slot opening into one of said first named slots, and a pair of supporting terminals at the divided base portion of said leg,

4. In induction heating apparatus, a device for applying a high frequency electromagnetic field to a'series of objects to be heated, comprising rigid substantially T-shaped conductor means formed with a series of openings along the cross portion of the T, for respectively receiving the objects, said openings being jointed by narrow slots, the leg of the T being divided by a narrow longitudinal slot opening into one oi said first 'shaped conductor means formed with openings arranged along the cross portion of the T, for respectively receiving the objects, such cross portion being longitudinally slotted between the openings, the leg of the T joining the cross portion at a region between two openings and such leg also being divided by a narrow longitudinal slot, slots in the leg and cross portions conjointly forming a T-shaped slot at said region, and a pair of supporting terminals at the divided base portion of said leg.

6. An inductor for applying a high frequency electromagnetic field to a series of objects to be heated, comprising rigid substantially T-shaped conductor means formed with a series of openings arranged along the cross portion of the T, for respectively receiving the objects, said openings being all joined by narrow slots, the leg of the T joining the cross portion at a region between two of the openings and such leg also being divided by a narrow longitudinal slot which in conjunction with the slot between the latter two openings, forms a T-shaped slot at said region, the divided base portion of said leg forming terminals for the connection of high frequency current. '7. An inductor for applying a high frequency electromagnetic field to a plurality of objects to be heated, comprising conductor means for extending with a scalloped iormation'along one side of said plurality of objects and then reversing to extend back again with a generally complementary scalloped formation along the other side of said plurality of objects, the protruding portions of the scalloped formation on one side being separated by narrow gaps from the corresponding portions on the other side, whereby a plurality of openings is provided with their walls adapted for closely spaced coupling relationship to the peripheries of the objects and for encircling the objects except at said gaps, the outside conductor walls being recessed at the regions of said protruding portions, and a metal cooling conduit extending around the inductor on its outside walls and soldered thereto, said conduit bridging the recessed portions of said walls.

PAUL E. DRAVNEEK. 

